Plasma torches were developed primarily as a high temperature heat source and are now widely used commercially for cutting, welding, coating and high temperature treatment of materials. Conventional direct current commercial plasma torches or guns include a pointed rod-like cathode generally formed of thoriated tungsten axially located within a bore in the body portion of the gun and an annular anode located downstream of the cathode having a nozzle orifice coaxially aligned with the cathode. A plasma-forming gas, typically argon or mixtures of argon and helium or argon and hydrogen, is introduced into the body portion of the gun such that the gas flows in an axial direction around the cathode and exits through the anode nozzle orifice. Plasma generation occurs in the gun in the arc region between the anode and cathode. The plasma is typically formed by initiating an arc between the anode and cathode using a high-frequency starting pulse, wherein the arc heats and ionizes the plasma gas to temperatures of about 12,000 degrees K. The heated and expanded plasma gas is then exhausted at high speed through the nozzle orifice. The gas flow through the gun can be axial or introduced in a manner so as to cause a vortex-type flow. The electrical characteristics of the plasma arc are determined by the gas flow rate, gas composition, anode nozzle orifice diameter and the electrode spacing.
Where the plasma gun is used for spraying a coating, the feedstock is usually in powder form suspended in a carrier gas and injected radially into the plasma effluent, either internally or externally of the nozzle exit depending on the gun manufacturer. Because the temperature drops off sharply in the plasma after it exits the anode nozzle, the powder is preferably introduced as close as possible to the point of plasma generation. U.S. Pat. No. 2,806,124 is an early disclosure of the basic principles of plasma technology and U.S. Pat. No. 3,246,114 includes an early disclosure of a commercial plasma gun.
Because of the geometry of a plasma gun and potential cathode deterioration, as discussed below, it is not possible to introduce the feedstock material axially through a conventional plasma spray gun, although the potential advantages have long been recognized. In a typical plasma jet coating apparatus, the feedstock powders are introduced radially into the plasma stream downstream from the plasma origin, either perpendicular to the axis or inclined in a direction with or counter-current to the flow of the plasma jet. As will be understood, the plasma interferes with particle penetration with a resistance that requires particle momentum sufficient to penetrate to the axis of the plasma jet. The particle momentum is provided by the carrier gas.
Further, thermal spray powders never have an absolutely uniform particle size and generally include a broad distribution of particle sizes. Carrier gas flow rate must further be adjusted dependent upon the particle size, wherein the smaller or lighter particles require a greater carrier-gas flow rate. Nevertheless, the particle injection velocity distribution will be broad even for a narrow particle size distribution and blends or mixtures of feed powders have very limited commercial applications. Therefore, heat and momentum transferred to the injected particles will vary over a wide range, resulting in a broad range of velocity and surface temperature distribution upon impact of the particles with the target or substrate. Because of the greater momentum of the larger or heavier particles, the larger particles will penetrate through the plasma jet and become entrained in the outer, colder gas region or ejected out of the plasma jet, resulting in unmelted fringe regions of the deposit coating. Very small or light particles of low momentum will fail to penetrate the plasma jet and will also be included in the fringe area. Very small particles which enter the plasma jet core may also overheat and vaporize. Therefore, only a fraction of the particles enter the core of the plasma jet and are deposited as a highly dense layer on the target substrate. The unmelted or partially melted particles may affect the density of the deposit. In a typical application, the deposition efficiency (i.e., the ratio of material fed into the plasma jet gun compared to the portion which actually forms the coating) is typically low, usually well below 70% for high melting materials, such as oxide ceramics and intermetallic compounds.
Unreactive gases, such as argon or helium, are employed as the plasma gas to avoid erosion or deterioration of the cathode electrode. As described above, the cathode is normally formed of thoriated tungsten and the electrode is operated at temperatures above 1000 degrees Centigrade. Diatomic gases, such as hydrogen or nitrogen, may be added to the inert plasma gas to enhance the power output of the plasma jet torch. However, reactive gases, such as oxygen, cannot be employed because reactive plasma gases would result in oxidation corrosion of the cathode. The use of reactive gases or reactive gas mixtures will cause the cathode to undergo local deterioration, thereby causing the cathode point of arc origination to wander, resulting in plasma arc instability or "arc wandering"; however, it would be desirable in a number of applications to utilize certain reactive gases, such as oxygen or oxygen bearing gas mixtures as the plasma forming gas. For example, certain plasma jet applications result in oxygen depletion of the feedstock. The utilization of oxygen, for example, as the plasma gas would result in restoration of oxygen in the resulting coating and eliminate the requirement of a post-spray oxygen replacement anneal.
It would also be very desirable to raise the operating power level of conventional plasma jet guns without decreasing energy efficiency or deterioration of the electrical components. In a typical plasma jet gun, the energy efficiency decreases as the operating energy level increases because of the inherently high electrical current operation and energy losses in the gun and power cables. Presently, energy is increased in a plasma jet gun by raising the current. Since the power input to a plasma jet gun is a product of the voltage and the current (Power=V.times.I), it would be desirable to raise the operating power level by increasing the plasma voltage rather than the current. Since the operating voltage is directly related to the plasma-forming gas used, as well as the cathode-anode spacing, it would be desirable to adjust these parameters for optimum operation. However, as described above, plasma forming gas selection is restricted to the group of unreactive or inert gases to avoid cathode deterioration. Cathode-anode spacing is limited due to the problems of initiating and maintaining stable plasma arc conditions with large interelectrode spacing.
Thus, the present plasma jet technology is limited in at least three important respects. First, radial injection of powdered feedstock results in poor deposition efficiency, reduced density of the deposit and requires a narrow range of feedstock particle size where uniform coatings are required. Second, reactive gases or reactive gas mixtures cannot be used as the plasma-forming gas to avoid deterioration of the cathode and arc wandering. Finally, the operating power level of conventional plasma jet guns cannot be significantly increased without decreasing the energy efficiency.
Various attempts have been made to avoid the problems of radial feed of plasma jet guns without commercial success. The principal solutions proposed by the prior art include (a) hollow cathode plasma guns, (b) RF (radio frequency) guns and (c) a plurality of plasma guns with a single feed. The hollow cathode gun, as the name implies, utilizes a hollow cathode tube, rather than a conventional rod-shaped cathode. The RF plasma gun employs a rapidly alternating electric field generated by a radio-frequency coil which replaces the arc as the plasma source. Although the hollow cathode and RF plasma guns have commercial promise, neither system has achieved commercial success.
As evidenced by U.S. Pat. No. 3,140,380 of Jensen, assigned to Avco Corporation, others have tried to merge two or more plasma effluents into a "joint plasma effluent into which a coating material is fed and reduced to substantially molten particles" for deposition on a substrate. In the prior art apparatus disclosed in the Jensen patent, a plurality of plasma guns or "plasma generating means" are "displaced symmetrically" with relation to a common axis such that the "plasma effluents are directed to intercept at a point and merged to form a joint plasma effluent." The plasma effluents from the individual plasma torches are then fed through a nozzle opening in the common axis and wire or powdered feedstock is fed through the nozzle opening in the common axis. As will be understood, this method of forming a "joint plasma effluent" does not result in a single or coalesced free-standing plasma and the impinging plasma effluent results in turbulence at the point of impingement through which the feedstock is fed. Further, the temperature of the plasma effluent at the point of impingement through which the feedstock is fed is substantially lower than the temperature of the plasma cores, resulting in lower efficiency than would be obtained for a true axial feed, wherein the feedstock particles are fed into the plasma core. This attempt to provide an axial feed for plasma spraying has not found commercial applications and the thermal spray industry therefore continues to utilize radial feed for plasma torches.
The prior art also includes other attempts to combine two or more plasmas as disclosed in U.S. Pat. No. 3,770,935 of Tateno, et al. In the plasma jet generator disclosed in the Tateno, et al patent, a positive plasma jet torch is aligned at a right angle to a negative plasma jet torch, such that the plasmas meet and function as a plasma jet torch of straight polarity to achieve a high arc voltage and improved efficiency. However, the plasma jet generator must utilize an inert plasma gas and radial feed of the feedstock. This system has not been introduced commercially and does not overcome the problems with radial feed as described above.
The prior art also includes numerous examples of transferred arc plasma guns or torches. Transferred arc plasma torches, wherein the substrate is connected electrically to the gun, has achieved commercial acceptance in many applications. It is also possible to utilize a second annular anode electrode, downstream of the primary anode, to transfer the plasma axially as disclosed in U.S. Pat. No. 2,858,411 of Gage. Transferred arc technology has not, however, resulted in a commercial axial feed plasma gun utilizing powdered feedstock, which is a primary object of the present invention.
Thus, although the problems of radial feed in commercial plasma spray apparatus have long been recognized, the prior art has failed to solve the problems described above in a commercially successful plasma spray system. There is, therefore, a long-felt need for an axial feed plasma spray system which has not been met by the prior art.